A critical comparison of polypropylene and polyurethane sling materials after implantation in a suburethral sheep model

Although polypropylene (PP) materials have been implanted for decades for urethral support in the pelvic floor, appropriate large animal models and advanced materials analysis techniques have not previously been used to investigate the clinical problems they can cause - inflammation, pain and erosion through tissues An ovine model duplicating the surgical procedure for suburethral sling surgery was developed. Here we present the results after 3 months implantation using immunohistochemistry and advanced materials characterisation of two materials PP and Polyurethane (PU). Both materials were well integrated into the tissue. The M1/M2 ratio in PP-implanted tissue was statistically significantly elevated (4.29) compared to PU (0.63) and control tissue (0.34). The higher ratio indicates a more inflammatory response to PP than PU. Surface roughness (assessed using atomic force microscopy) increased in both materials, Rq from 5.73-10.2nm in PP and from 1.03-2.96nm in PU; whilst Ra went from 4.75-7.85nm in PP and from 0.81-2.36nm in PU. Notably, surface stiffness increased by 0.05GPa in PP and decreased by 0.2GPa in PU. PP underwent both surface and bulk material degradation, PU did not. Detailed testing of implantable materials in an appropriate animal model should be conducted before materials are introduced into clinical practice. It is salutary that this has never been reported before. The use of material characterisation techniques allowed us to identify problems in the performance of PP, notably surface degradation, changes in bulk properties and stiffening, which can activate macrophages. In contrast, PU appears a more suitable alternative material for use in treating patients with SUI.